Over the last few years, calcium has been emphasized as having a possible role in cell death produced by a number of toxic chemicals. Several studies now suggest that ionized calcium dramatically increases in hepatocytes exposed to a cytotoxic compound. This project will examine the role of this increase of ionized calcium in the cytoplasm produced by exposure of the cell to halogenated hydrocarbons (carbon tetrachloride, 1, 1- dichloroethylene, bromobenzene). Studies proposed will attempt to further validate primary cultures of hepatocytes as an appropriate model for these studies. Validation will be by examination of the role of metabolic activation of CC14 to chemically reactive intermediates and modification of activation by chemical alteration of the mixed function oxidase system. Studies with the model system will also characterize a discrepancy between the effects of CC14 on calcium homeostasis in vivo and in the tissue culture system. Another line of investigation will define a possible consequence of the increase of cytoplasmic calcium produced by halogenated hydrocarbon exposure. Activation of a calcium-sensitive endonuclease appears to play an important role in programmed cell death (apoptosis), and this will be examined in rat liver and monolayer cultures of hepatocytes after halogenated hydrocarbon exposure. Another major effort of the project will be to isolate and characterize mutant cell populations resistant to cytotoxicity produced by halogenated hydrocarbons. This effort will attempt to identify enzymatic systems critical in expression of cytotoxicity, or critical to protection of the cell from the cytotoxic effects of chemically reactive metabolites of the halogenated hydrocarbons.